1. Field of the Invention
The present invention relates to an echo canceller for improving a speech quality etc. in a telephone network etc.
2. Description of the Related Art
Echo cancellers are installed for such purposes as preventing the deterioration of a speech quality attributed to echoes which occur in, for example, 2-wire/4-wire converting hybrid devices included in various telephone networks etc. in order to mutually switch 2-wire transmission lines and 4-wire transmission lines.
Heretofore, echo cancellers have been often employed for suppressing echoes which occur in networks of long transmission lines and great transmission delays, such as a satellite communication network, etc.
Further, in recent years, techniques in which a transmission band is compressed by a voice CODEC (voice coder/decoder) so as to reduce the transmission rate of voice have extensively come into practical use, and echo cancellers have been extensively applied in order to eliminate echoes ascribable to processing delays which are involved in such CODECS. Concretely, the echo cancellers have come into wide use in radio networks such as the base station system of mobile terminals, etc.
Besides, as a telephone network and an ATM (asynchronous transfer mode) network are integrated more in the future, the necessity of echo cancellers will rise due to the increases of processing delays attendant upon the disassembly/assembly of ATM cells, etc.
FIG. 1 is a diagram for explaining the applied positions of echo cancellers for suppressing echoes which occur in 2-wire/4-wire converting hybrid devices (hereinbelow, simply termed "hybrid devices").
Referring to FIG. 1, the echo of a speaker "A" signifies a signal which is so produced that part of the voice of the speaker "A" leaks into the reception line of the speaker "A" in the hybrid device (Hybrid) 101(#B) on the side of a speaker "B". The echo of the speaker "A" is suppressed by the echo canceller (EC) 102(#A) (hatched part) which is installed on the speaker-B side. A path which extends from the echo canceller 102(#A) back to the same 102(#A) via the hybrid device 101(#B) on the speaker-B side is called the "echo path" 103 of the echo canceller 102(#A). In addition, with respect to the echo canceller 102(#A), the speaker "A" is called a "far-end speaker", and the speaker "B" is a "near-end speaker".
Conversely, the echo of the speaker "B" signifies a signal which is so produced that part of the voice of the speaker "B" leaks into the reception line of the speaker "B" in the hybrid device 101(#A) on the speaker-A side. The echo of the speaker "B" is suppressed by the echo canceller (EC) 102(#B) which is installed on the speaker-A side. Although not especially shown in the figure, the echo path of the echo canceller 102(#B) extends from the echo canceller 102(#B) back to the same 102(#B) via the hybrid device 101(#A) on the speaker-A side. In addition, a far-end speaker with respect to the echo canceller 102(#B) is the speaker "B", while a near-end speaker is the speaker "A".
As delays involved between the speakers "A" and "B" are longer, a transmission delay to occur therebetween increases more. Accordingly, the speaker "A" comes to hear later the echo arising from his/her own voice and to more conspicuously perceive the echo as being offensive to the ear.
FIG. 2 is a block diagram showing the prior-art construction of the echo canceller 102 depicted in FIG. 1.
Generally, in the echo canceller 102, a tap coefficient updating unit 204 successively updates N tap coefficients exhibiting the characteristics of the echo path 103 and successively held in a tap coefficient memory 203, on the basis of residual echo signals 212 successively outputted from a subtracter 210 and receive-in signals 205 successively held in a tap memory 202. It holds the updated tap coefficients in the tap coefficient memory 203 anew.
Besides, an adaptive FIR (Finite Impulse Response) filter 201 executes a convolution calculation for the receive-in signals 205 successively held in the tap memory 202 and the N tap coefficients successively updated by the tap coefficient updating unit 204, thereby to generate a pseudo or artificial echo signal 209. That is, the adaptive FIR filter 201 is a filter which realizes the characteristics of the echo path 103. Here, the echo path 103 is assumed to have linear characteristics, which are estimated as an impulse response. Further, the adaptive FIR filter 201 is implemented as a transversal filter which executes the convolution calculation of a finite impulse response (FIR) approximating the impulse response of the linear characteristics.
The tap coefficient updating unit 204 calculates the updated values of the respective values of the N tap coefficients from the residual echo signals 212 outputted from the subtracter 210 and the receive-in signals 205 successively held in the tap memory 202, every sampling point and on the basis of, for example, an algorithm called "learning identification". Subsequently, using the N updated values, the tap coefficient updating unit 204 updates the respective values of the N tap coefficients calculated at the last sampling point and held in the tap coefficient memory 203. Also, the tap coefficient updating unit 204 outputs the resulting N tap coefficients to the coefficient setting portion of the adaptive FIR filter 201 and holds them in the tap coefficient memory 203 anew.
Here, since a conventional echo canceller assumes white noise as the receive-in signal, the echo cannot be completely removed in a situation where background noise enters the actual voice signal or the near-end speaker side. In particular, when the transmission delay of the echo path increases, the influence thereof becomes conspicuous. As shown in FIG. 2, therefore, a processor 211 called "NLP (nonlinear processor)" is inserted on the output side of the subtracter 210 included in the echo canceller 102. The NLP 211 executes such a process that, if the level of the residual echo signal 212 outputted from the subtracter 210 does not exceed a certain level, the signal is forcibly made zero by way of example. Accordingly, the process is a kind of nonlinear process. However, in a case where the speakers "A" and "B" are talking at the same time or where the near-end speaker is talking, the state is detected by a superposed-talk detecting circuit or the like not especially shown, and the operation of the NLP 211 is stopped.
As stated before, it is premised for the echo canceller 102 that the echo path 103 has-the linear characteristics, which can be simulated by the adaptive FIR filter 201. Herein, the tap length of the adaptive FIR filter 201 is set at a length which can cover the maximum time period supposable as the transmission delay of the echo path 103, in consideration of a network to which the echo canceller 102 is connected. A time period corresponding to the length is called an "echo control time". More concretely, the tap length of the adaptive FIR filter 201 is usually set at a time length which is obtained in such a way that an impulse response time arising in the hybrid device 101 is added to the maximum transmission delay time of the echo path 103.
By way of example, in a telephone network laid within the State of Japan, the maximum delay time of the echo path 103 is said to be on the order of 40-50 [msec] in and around Tokyo. It is known that the echo does not offend the ear within such limits.
Recently, especially in a mobile-type network such as PHS network or portable telephone network, etc., an overall transmission delay involved in the network tends to increase due to the increases of transmission delays developing in a radio line portion, a voice CODEC and a line multiplexer/demultiplexer which are included in the network. In the mobile-type network, therefore, echo cancellers have come to be often inserted into a base station etc.
Problematic here are the following two points:
1. Let's consider a case shown in FIG. 3 where an STM (synchronous transfer mode) network 301 and an ATM (asynchronous transfer mode) network 302 which are mobile-type networks, and an STM network 303 which is a stationary-type network are connected through cell assembling/disassembling units 304 and 305, etc. The transmission delay time of the echo path 103 tends to increase as viewed from the echo canceller 102 which is installed in the base station or the like. Consequently, there might occur a situation where the transmission delay time of the echo path 103 exceeds the echo control time of the echo canceller 102.
2. The echo cancellers 102 are installed in, e. g., the mobile-type network, etc. anew. Accordingly, there might appear the tandem connection of at least two echo cancellers 102, such as the connection of the echo canceller 102 on the side of the echo path 103 and the echo canceller 102 on the side of a transit trunk as shown in FIG. 4.
Originally, the designer of a network ought to sufficiently grasp the positions and characteristics of echo cancellers 102 which are installed in the network. In actuality, however, he/she is sometimes difficult of fully grasping the circumstances of the network.
In the above case-1, the echo canceller 102 cannot operate normally, to pose such a problem that noise occurs.
In the above case-2 as exemplified in FIG. 4, the NLP 211 for the nonlinear operation exists in the echo canceller 102 on the echo path side, and hence, the echo path 103 no longer has the linear characteristics as viewed from the echo canceller 102 on the transit trunk side. Therefore, the echo canceller 102 on the transit trunk side fails to generate the optimum pseudo echo signal 209 (refer to FIG. 2), and such a problem as the occurrence of noise might be similarly posed.